Envelope morphogenesis affecting cell shape in Caulobacter comprises equatorial constriction that culminates in cell fission, and outgrowth at one pole, as stalk development. The site of polar outgrowth is first occupied by a flagellum, whose basal apparatus includes a plate that remains with the outer membrane after flagellum release. The plate becomes the leading tip of the developing stalk. Cell populations can be directed almost quantitatively into one or the other developmental pathway by manipulation of the ratio of phosphate:carbon in the growth medium. The precise critical ratio will be determined by measuring the response of carbon-limited populations to various levels of phosphate. The activities and substrate requirements of protein kinases will be assayed to determine whether phosphorylation of specific enzymes is involved in the morphogenetic responses to phosphate concentrations. Low P:C ratios precipitate the shedding of stalks by "stalk-abscission" mutants, a process accompanied by flagellum formation at the opposite pole. The possibility that organization of the flagellar plate is sufficient stimulus for flagellum formation and subsequent stalk development will be studied by examination of the flagellar basal site an developing stalk tips in gently lysed cells of wild-type strains, abscission mutants, and mutants capable of developing stalks at random sites over the cell surface. The aim of these studies is to identify metabolic and structural mediators of surface morphogenesis in these bacteria.